Page 60 BETTER FRUIT March 



INJURY CAUSED BY THE APPLE POWDERY MILDEW 



BY W. H. VOLCK, WATSONVILLE CALIFORNIA (Continued from February Number) 



We have frequently observed that 

 when arsenical spraying is done early in 

 the season (shortly after the blossoms 

 have fallen) the foliage may become an 

 especially dark green color and new 

 growth develop with more than ordinary 

 vigor. The effect is apparently due to 

 arsenic stimulation, and where it is pro- 

 nounced the trees outgrow the mildew 

 for a time. Frequently this stimulation 

 terminates in poisoning, and the mildew 

 may then obtain a stronger hold than 

 ever. Stimulation with partial subjuga- 

 tion of the mildew has been observed, 

 and after spraying with zinc arsenite, 

 arsenite of lime and arsenate of lead. 



The above mentioned compounds and 

 mixtures by no means exhausts the list 

 of those that might be tried, and possibly 

 would prove specifics against the mil- 

 dew. The results obtained in these 

 experiments, however, would indicate that 

 the ultimate discovery of a substance 

 that, when applied as a spray, will kill 

 the mildew upon the stems is very 

 improbable. So far purely curative treat- 

 ments have proved very unpromising, as 

 ail substances powerful enough to kill 

 the well established fungus were injuri- 

 ous to the plant. 



Those substances which partly sub- 

 jected the mildew without materially 



injuring the foliage, or even to an extent 

 stimulated healthy growth, offer the most 

 encouragement. The problem of spray- 

 ing for the mildew is somewhat similar 

 to that of applying arsenicals. In both 

 cases the spraying must be done without 

 introducing objectionable substances into 

 the tissue of the plant. With arsenicals 

 this has been accomplished by using 

 arsenic compounds which are insoluble 

 in water, and up to the present time the 

 best results with the mildew have been 

 obtained by following a like idea with 

 the sulphur compounds. 



Sulphur in the form of sulphides and 

 the pure element has long been regarded 

 as a good remedy for mildews. Many 

 mildews yield to a simple dusting with 

 flowers sulphur, especially in the warm 

 climates. Where dusting proved inef- 

 fective liquid spraying with sulphur has 

 often been resorted to. These liquid 

 applications are of two kinds, those con- 

 taining insoluble sulphur in suspension 

 and those in which the sulphur is in solu- 

 tion in the form of sulphides. The sol- 

 uble sulphides are combinations of sul- 

 phur with alkalis, such as caustic lime, 

 caustic soda and caustic potash. When 

 sulphur in the insoluble form is applied 

 as a liquid spray the advantage over 

 dusting is probably that of more thor- 



ough distribution and better sticking 

 properties. Soluble sulphur, on the other 

 hand, has a more powerful and imme- 

 diate action. 



Sulphur in the free state apparently 

 acts by means of its vapor, while the 

 soluble sulphides have both vaporization 

 and water solution as a means of action. 

 The soluble sulphides fail to give satis- 

 faction largely because of their over 

 energetic immediate action, which causes 

 plant injury. When used dilute enough 

 to avoid injury their prolonged action is 

 very slight, because the deposit of sul- 

 phur is so small, hence failure to control 

 the mildew. 



The soluble sulphides failed because of 

 their solubility, and it occurred to the 

 writer that something might be accom- 

 plished with sulphide sulphur combined 

 in an insoluble form. There are several 

 such sulphides, and the ones experi- 

 mented with have already been men- 

 tioned, namely, those of copper and iron. 

 These insoluble sulphides brought about 

 marked control of the mildew with very 

 little toxic effect on the plant. With iron 

 sulphide there is apparently a stimula- 

 tion of the plant which supplements the 

 fungicide action. The composition and 

 action of these insoluble sulphides will 

 be discussed more fully later. 



The small experiments to control the 

 mildew, described in the preceding para- 

 graphs, pointed out the lines upon which 

 more extensive control work might be 

 undertaken with greatest hope of suc- 

 cess. These experiments were conducted 

 during the spring and summer of 1908, 

 and are given in detail as follows: 



The trees included in this and the fol- 

 lowing experiments were located, in the 

 C. H. Rodgers orchard, situated in the 

 Pajaro Valley, about one mile from 

 Watsonville. This orchard comprises 

 100 acres, now all in apple. A consider- 

 able portion of the trees are 18-year-old 

 Newtowns, a smaller number, including 

 the White Winter Pearmains, are over 

 30 years old, and another large portion 

 is in Newtowns eight years old. The 

 experiments were each distributed so as 

 to include these three portions of the 

 orchard, and, in addition, some young 

 Bellfliower trees were also treated. 



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